Many components in gas turbines are not only subject to thermal, mechanical and erosive stresses but also to corrosive influences to a considerable extent. Causes of corrosion can be found in the type and source of the fuel and the composition of the combustion air. The temperature in the corrosion exposed area is a contributing factor.
To protect against corrosion, blades are usually coated with either diffusion or overlay coating. These coatings are both expensive and at low temperature inductile which may cause cracking. The coating cracks can then create crack initiation sites for the base material leading to premature failure. Due to the lower temperature within the blade internal cooling passages this problem can be more acute.
SU 1615055 A1 describes a screw propeller, comprising a set of hub sectors made monolithic with blades. The hub is applied to a stainless steel corrosion prevention sleeve enclosing a propeller shaft.
US 2005/0118024 describes throughflow openings for a cooling medium in a coolable component. The throughflow opening comprises an insert that reduces the size of the first opening cross-section to a second opening cross-section, and that is released from the first opening if the second opening cross-section becomes blocked as a result of a local temperature rise and a thermally unstable joining between the insert and the component, being mounted in a first opening.
U.S. Pat. No. 6,709,771 B2 describes a hybrid component like a blade of a gas turbine engine that may be cast as monolithic structure with internal cooling channels. A single crystal airfoil forms part of a mould where a ceramic insert is positioned prior to filling the mould with powder metallurgy material. The ceramic insert defines during the casting process the cooling channels and is later dissolved to create the open cooling channels within the cast component.